1. Field of the Invention
The present invention relates to laser modulators and more particularly to an absorption cell for modulating two input laser beams to produce an output beam including dark pulses.
2. Description of the Prior Art
There exist many devices in the prior art which accept two or more input laser beams and perform frequency conversion or difference-frequency mixing functions.
An example of a prior art device of this nature is described in U.S. Pat. No. 3,914,618 issued Oct. 21, 1975 on an application filed Jan. 28, 1974 by S. E. Harris and entitled TWO-PHOTON PUMPED FREQUENCY CONVERTER AND METHOD. This patent describes a frequency converter having a cell containing a metal vapor which is pumped by a first laser beam at a frequency .omega..sub.p such that two of its photons equals the spacing of a two-photon transition of the metal vapor to symmetrically excite the metal atoms. The cell is also simultaneously responsive to an incident radiation beam at frequency .omega..sub.t which may be another laser beam. The second radiation beam couples the two-photon excited state to an allowed transition to ground. This produces or generates side bands at twice the pump frequency 2.omega..sub.p, plus or minus the frequency of the second beam .omega..sub.t for an output frequency 2.omega..sub.p .+-. .omega..sub.t.
Another typical example of producing a difference frequency by using two laser beams is described in U.S. Pat. No. 3,869,618 issued Mar. 4, 1975 and an application filed Oct. 30, 1972 by Lax et al entitled HIGH-POWER TUNABLE FAR INFRARED AND SUBMILLIMETER SOURCE and assigned to Massachusetts Institute of Technology. This patent described the use of two high-intensity laser beams, both in the near-infrared region or visible region to produce a tunable output signal. The frequency of the output beam is the same as the difference in frequency between the two input beams. The two beams are directed upon a non-linear crystal which serves to mix the input radiation and thereby generate a difference-frequency radiation that is emitted from the crystal as an output.
Neither of the aforesaid references, or other known prior art anticipate, suggest or relate to the present invention wherein mode-locked laser pulses are used to modulate a continuous wave laser beam within a two-photon absorption cell to produce dark pulses. The device of the present invention functions as a shutter for the continuous wave beam rather than a mixing or frequency conversion device, with the result being the production of dark pulses as output from the cell.